I would pay the dealership to do it because the proper way is to have a machine and hook it up to the upper rad hose, and the radiator, so you're actually flushing out the old stuff and putting new stuff in.

Without that machine, it would seem that you can't get all the old coolant out, and imho the car is worth it as it's one of the cheaper services....

If you could get it level it would be great but you wont get it all out still with like mention above. With the coolant in the water pump, t-stat and engine block it all wont come out from the drain plug on the bottom of the radiator.
But imo ramps would do just fine if your looking to rejuvenate your fluid. You'll get out maybe 90% and top the rest off with fresh stuff. Dont forget to bleed it when you're done.

I have BMW's coolant flush procedure from the TIS. It calls for no special flushing agent, nor the use of a coolant transfer machine. The TIS procedure is open the radiator drain and pull the lower radiator hose to the water pump. The DIY section has a few posts on how to perform the service and all are quite good at explaining the procedure. All I've done for my 193K mile E90 is drain and refill the coolant.

I was looking at flushing with Prestone super radiator cleaner or the super flush. Is there any benefit from one over the other?

Bare in mind you have a magnesium block, best not to have any untested chemicals in there (it's not like you're going to have any rust from an iron block to clean out, which those flushes are mostly designed for.) And BMWs have a reputation for being super fussy about the coolant that's used.

I used some low-pressure, 15-20 lbs, shop air to remove more coolant, it's documented in the DIY.